Accommodation, as it relates to the human visual system, refers to the ability of a person to use their unassisted ocular structure to view objects at both near (e.g. reading) and far (e.g. driving) distances. The mechanism whereby humans accommodate is by contraction and relaxation of the cilliary body which inserts into the capsular bag surrounding the natural lens. Under the application of cilliary stress, the human lens will undergo a shape change effectively altering the radius of curvature of the lens. This action produces a concomitant change in the power of the lens. However, as people grow older the ability for them to accommodate reduces dramatically. This condition is known as presbyopia and currently affects more than 90 million people in the US. The most widely believed theory to explain the loss of accommodation was put forth by Helmholtz and states that as the patient ages, the crystalline lens of the human eye becomes progressively stiffer prohibiting deformation under the applied action of the cilliary body.
People who can see objects at a distance without the need for spectacle correction, but have lost the ability to see objects up close are usually prescribed a pair of reading glasses or magnifiers. For those patients who have required previous spectacle correction due to preexisting defocus andor astigmatism the patient is prescribed a pair of bifocals, trifocals, variable, or progressive focus lenses that allow the person to have both near and distance vision. Compounding this condition is the risk of cataract development as the patient ages. In fact, cataract extraction followed by intraocular lens (IOL) implantation is the most commonly performed surgery in patients over 65 years old (reference).
To effectively treat both presbyopia and cataracts the patient can be implanted with a multifocal IOL. The general concepts and designs of multifocal IOLs have been described before in the ophthalmic and patent literature. The simplest design for a multifocal IOL is commonly referred to as the “bull's eye” configuration and comprises a small, central add zone (1.5 mm to 2.5 mm in diameter) that provides near vision (“Intraocular Lenses in Cataract and Refractive Surgery,” D. T. Azar, et. al., W. B. Saunders Company (2001); “Intraocular Lenses: Basics and Clinical Applications,” R. L. Stamper, A Sugar, and D. J. Ripkin, American Academy of Ophthalmology (1993), both of which are hereby incorporated herein by reference). The power of the central add zone is typically between 3 to 4 diopters greater than the base power of the IOL, which translates to an effective add of 2.5 to 3.5 diopters for the entire ocular system. The portion of the lens outside the central add zone is referred to as the base power and is used for distance viewing. In theory, as the pupil constricts for near viewing, only that central add zone of the lens will have light from the image passing through it. However, under bright viewing conditions the pupil will also constrict leaving the patient 2 to 3 diopters myopic. This can be potentially problematic for a person who is driving in a direction with the sun shining straight at them, e.g. driving west around the time of sunset. To counteract this problem, an annular design with the central and peripheral portion of the lens designed for distance viewing and a paracentral ring (2.1 to 3.5 mm) for near vision. This design will maintain distance viewing even if the pupil constricts (Intraocular Lenses in Cataract and Refractive Surgery, D. T. Azar, et. al., W. B. Saunders Company (2001); “Intraocular Lenses: Basics and Clinical Applications,” R. L. Stamper, A Sugar, and D. J. Ripkin, American Academy of Ophthalmology (1993), which is hereby incorporated herein by reference). The most widely adopted multifocal IOL currently sold in the US is described in U.S. Pat. No. 5,225,858, which is hereby incorporated herein by reference. This IOL is known as the Array lens and comprises five concentric, aspheric annular zones. Each zone is a multifocal element and thus pupil size should play little or no role in determining final image quality.
However, as with standard intraocular lenses the power and focal zones of the lenses must be estimated prior to implantation. Errors in estimating the needed power as well as shifting of the lens post-operatively due to wound healing often results in less than optimal vision. The latter effect is particularly problematic for the case of the bull's eye lens if a transverse (perpendicular to the visual axis) shift of the IOL occurred during healing. This would effectively move the add part off the visual axis of the eye resulting in the lost of desired multifocality. The Array and paracentral IOL designs can partly overcome the dislocation problem during wound healing-although any IOL movement longitudinally (the direction along the visual axis), preexisting astigmatism, or astigmatism induced by the surgical procedure can not be compensated using these multifocal IOL designs. This results in the patient having to choose between additional surgery to replace or reposition the lens or to use additional corrective lenses.
A need exists for an intraocular lens which can be adjusted post-operatively in vivo to form a multifocal intraocular lens. This type of lens can be designed in-vivo to correct to an initial emmetropic (light from infinity forming a perfect focus on the retina) state and then the multifocality may be added during a second treatment. Such a lens would remove some of the guess work involved in presurgical power selection, overcome the wound healing response inherent to IOL implantation, allow the size of the add or subtract zone(s) to be customized to correspond to the patient's magnitude and characteristics of dilation under different illumination conditions, and allow the corrected zones to be placed along the patient's visual axis.